FCIQMC-Tailored Distinguishable Cluster Approach: Open-Shell Systems

A recently proposed
tailored approach based on the distinguishable
cluster method and the stochastic FCI solver, FCIQMC [J. Chem.
Theory Comput. 2020, 16, 5621], is extended to open-shell
molecular systems. The method is employed to calculate spin gaps of
various Fe­(II) complexes, including a Fe­(II) porphyrin model system.
Both distinguishable cluster and fully relaxed CASSCF natural orbitals
were used in this work as reference for the subsequent tailored distinguishable
cluster calculations. The distinguishable cluster natural orbitals
occupation numbers were also used as an aid to the selection of the
active space. The effect of the active space sizes and of the explicit
correlation correction (F12) onto the predicted spin gaps is investigated.
The tailored distinguishable cluster with singles and doubles yields
consistently more accurate results compared to the tailored coupled
cluster with singles and doubles.

近期，一种基于可区分簇方法和随机FCI求解器FCIQMC [J. Chem. Theory Comput. 2020, 16, 5621] 的定制化方法被拓展应用于开放壳分子系统。此方法被应用于计算多种Fe(II)配合物的自旋隙，包括Fe(II)卟啉模型系统。在此研究中，既采用了可区分簇方法，也运用了完全松弛的CASSCF自然轨道作为后续定制化可区分簇计算的参照。此外，可区分簇自然轨道的占据数还被用作选择活性空间的辅助工具。本研究还对活性空间大小和显式相关校正（F12）对预测自旋隙的影响进行了探究。相较于定制化耦合簇方法，定制化可区分簇方法在单电子和双电子计算中能够提供更为精确的结果。